The present invention relates generally to electro-optic devices and more particularly, to an optical sub-assembly for interfacing electro-optical devices with optical fibers.
The proliferation of optical communication networks intended for subscribers has created a strong demand for low-cost and compact optical assemblies. One factor that increases cost is the necessity of precision alignment between the end of an optical fiber and an optoelectronic device. The precision of alignment that is required between the end of an optical fiber and an optoelectronic device via a reflector varies with application.
For example, on the receiving side of an optical communication system, a received optical signal is optoelectrically converted into an electrical signal by a photodetector such as a photodiode, and information is reproduced according to the electrical signal obtained. Alignment difficulties on the receiver side of an optical communication system may be introduced by characteristics of the optical fiber, the photodetector, and a reflector reflecting the optical signal from the optical fiber to the photodetector.
The alignment difficulty may generally be addressed by making a detector xe2x80x9cartificiallyxe2x80x9d larger than it needs to be, resulting in slower photodetectors with inherently larger rise times, fall times, and settling times. Larger photodetectors may therefore limit system level bandwidth which ultimately limits transmission data rates. The bandwidth of a photodetector is generally determined by the transit time of the photo-generated carriers in the absorption region and the RC time constant. The inherently lower bandwidth, for larger photodetectors, is caused by higher shunt resistance and larger shunt capacitance of the photo conductive areas of the detectors. More rapid response requires a smaller electrostatic capacitance at the depletion layer. The electrostatic capacitance decreases with decreasing depletion region area. Therefore, the diameter of the light receiving portion of high speed photodetectors are typically restricted to minimize the capacitance of the device.
However, optical beams emanating from an optical fibers are typically divergent and have a relatively wide cross-sectional area that requires a wide depletion region. For smaller detectors, the reflected beam may miss the active area of the detector altogether, giving rise to low coupling or no coupling. Moreover, the size of the reflected beam makes it necessary to perform a relatively difficult active alignment in the x, y and z planes. It would therefore be advantageous to provide a compact, high speed optical subassembly to efficiently couple light into a photodetector. It would also be advantageous to provide an optical sub-assembly that simplifies alignment between the fiber and the photodetector.
Further, light beams emanating from a fiber often travel through v-grooves of an optical sub-assembly. Currently used v-grooves, which are also produced by etching from the [100] crystallographic plane, may clip some light, resulting in low coupling. Therefore, it would be advantageous to provide an optical sub-assembly with v-grooves that do not clip light from the optical fiber.
In an exemplary embodiment of the present invention, an optical receiver includes a photodetector adapted to receive an incoming optical signal from a fiber. The photodetector is coupled to a silicon substrate with a top surface cleaved from a [100] silicon crystallographic plane by about 5.7xc2x0 to 9.7xc2x0 and a reflector extending from the top surface.
The reflector is created by etching into the silicon substrate from the top surface along a [111] silicon crystallographic plane. The reflector has an angle of about 45xc2x0 to about 49xc2x0 relative to the top surface. A light beam emanating from the fiber is reflected off of the reflector onto the photodetector.
The present invention is also directed to a method for manufacturing an optical receiver. In an exemplary embodiment, the method involves cleaving a silicon substrate having a top surface along a [100] silicon crystallographic plane by about 5.7xc2x0 to about 9.7xc2x0 to form an off-axis top surface of an optical sub-assembly. The off-axis top surface is etched along a [111] silicon crystallographic plane to form a reflector having an angle of about 45xc2x0 to about 49xc2x0 relative to the off-axis top surface.
It is understood that other embodiments of the present invention will become readily apparent to those skilled in the art from the following detailed description. The described embodiments of the invention illustrate the best modes contemplated for carrying out the invention. As it will be realized, the invention is capable of other and different embodiments and the details are capable of modification in various other respects, all without departing from the spirit and scope of the present invention. Accordingly, the drawings and detailed description are to be illustrative in nature and not restrictive.